With development of communications technologies, a transmission rate of a high-speed optical fiber transmission system develops from 40 Gbit/s to 100 Gbit/s, or even to 400 Gbit/s, a coherent receiving technology is widely applied. A main structure of a coherent receiving system is shown in FIG. 1. At a transmit end, after a digital signal code stream undergoes forward error correction coding performed by a forward error correction coding module, and phase modulation performed by a phase modulation module, the digital signal code stream is sent to an optical fiber link. At a receive end, a receiving module performs coherent receiving by using the optical fiber link and performs demodulation to recover an analog baseband signal; an analog-to-digital conversion module samples the analog baseband signal, to obtain a multi-level digital signal; an equalization module performs time-domain and frequency-domain equalization on the multi-level digital signal; then, a phase estimation and recovery module performs carrier phase estimation (CPE) and recovery, on the equalized multi-level digital signal; and finally, a decision decoding module performs decoding and decision, to obtain a recovered digital signal code stream.
However, for the foregoing coherent receiving system and a similar communications system, a problem of a phase jump often occurs in the signal on which the carrier phase estimation and recovery are performed, that is, a 90-degree phase jump, −90-degree phase jump or a 180-degree phase jump occurs in a data segment of the signal obtained after the carrier phase estimation and recovery. For a communications system, a phase jump is unacceptable, because the phase jump causes continuous burst bit errors, and the segment of continuous burst bit errors greatly affects or even invalidates a function of forward error correction decoding, so that a great number of bit errors occur in the communications system.